Remediation of hydrocarbon-contaminated soils by ex

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Environmental Technology, 2014 http://dx.doi.org/10.1080/09593330.2014.902109

Remediation of hydrocarbon-contaminated soils by ex situ microwave treatment: technical, energy and economic considerations P.P. Falciglia∗ and F.G.A. Vagliasindi Department of Civil and Environmental Engineering, University of Catania, Viale A. Doria, Catania, 6 – 95125, Italy

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(Received 11 October 2013; accepted 3 March 2014 ) In this study, the remediation of diesel-polluted soils was investigated by simulating an ex situ microwave (MW) heating treatment under different conditions, including soil moisture, operating power and heating duration. Based on experimental data, a technical, energy and economic assessment for the optimization of full-scale remediation activities was carried out. Main results show that the operating power applied significantly influences the contaminant removal kinetics and the moisture content in soil has a major effect on the final temperature reachable during MW heating. The first-order kinetic model showed an excellent correlation (r 2 > 0.976) with the experimental data for residual concentration at all operating powers and for all soil moistures tested. Excellent contaminant removal values up to 94.8% were observed for wet soils at power higher than 600 W for heating duration longer than 30 min. The use of MW heating with respect to a conventional ex situ thermal desorption treatment could significantly decrease the energy consumption needed for the removal of hydrocarbon contaminants from soils. Therefore, the MW treatment could represent a suitable cost-effective alternative to the conventional thermal treatment for the remediation of hydrocarbon-polluted soil. Keywords: diesel fuel; ex situ soil treatment; hydrocarbon; microwaves; thermal desorption

1. Introduction Due to the fast development of petroleum industry, a large amount of hydrocarbon products have been released into the soil environment via spills, leakage or incidents.[1] Among hydrocarbons, petroleum-derived diesel is composed of approximately 75% saturated hydrocarbons (primarily paraffins, including n-, iso- and cycloparaffins) and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes),[2] and it is widely used in the world and represents a permanent source of soil pollution.[3–5] Diesel-contaminated soil is unsuitable for agricultural, residential or recreational uses and represents an ecological disaster and an economic loss.[5] Several chemical–physical or biological treatments have been reported [3,5,7–12] to remedy diesel pollution. However, these treatments may be ineffective, too expensive or lengthy.[6] Ex situ conventional thermal desorption was also successfully applied to remedy contaminated soil presenting an excellent contaminant removal percentage in a very short remediation time [13–16] but it may be expensive due to the energy costs. Microwaves (MWs) were initially adopted for communication purposes and for several decades they have been applied as a cost-effective alternative to current heating technologies for many other applications, such as materials (foods, polymers, wood and ceramics) processing or mineral treatment.[17] In recent years, thermal remediation ∗ Corresponding

author. Email: [email protected]

© 2014 Taylor & Francis

using MW heating has attracted great attention in the environmental field, i.e. waste treatment and polluted soil remediation, since it offers the potential to significantly reduce treatment times, risk of contamination and costs due to the direct interaction of MWs with the soil and its ability to overcome heat and mass transfer limitations.[18] MWs are a separate band of electromagnetic radiation with frequencies in the range of 300 MHz to 300 GHz. The key factor of the remediation process is represented by the mechanism due to a partial dissipation of the electromagnetic field energy and its conversion into heat necessary for the thermal desorption of the contaminants. In fact, the internal temperature distribution of a material, such as the soil, using conventional heating, is limited by its thermal conductivity, whereas in the case of MW radiation, the alternating electromagnetic field induces the rotation of the dipoles of water and other polar substances present in the soil. The intermolecular friction results in the generation of heat.[19] Moreover, MWs are absorbed by materials with a high dielectric loss factor (absorbing), while passing through the low-loss (transparent) material, resulting in a selective, uniform and rapid heating. Therefore, heating duration can be significantly reduced compared with those required when using conventional heating methods.[20] The majority of the absorbed MW power is converted to heat within the materials and the rate of heat generated

2

P.P. Falciglia and F.G.A. Vagliasindi

(T t −1 (◦ C min−1 )) depends directly on the frequency of the applied electromagnetic field and on the dielectric properties of the treated medium [21]:

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P ω · ε0 · ε  |E|2 T = = , t cp · ρ cp · ρ

1

(1)

2

where P is the power absorbed per unit volume (W m−3 ); ω is angular frequency (ω = 2πf , f MW frequency (Hz)); ε0 is permittivity of free space (8.85 10−12 F m−1 ); ε  is dielectric loss factor (–); E is internal electric field (V m−1 ); cp is heat capacity of the medium (kJ kg−1 ◦ C−1 ) and ρ is density of the medium (kg m−3 ). ε that is the real part of the complex permittivity denotes the electric energy storage capacity of the medium, while ε  that is the imaginary parts of the complex permittivity can be considered as the ability of the medium to convert electromagnetic energy into heat, due to the dielectric polarization of the particles in an alternating electric field. The dielectric properties are also important parameters in determining the penetration depth (Dp ), which represents the ability of the electromagnetic waves to penetrate into the medium. In particular, Dp is defined as the distance from the emission point at which the power (P) drops to 0.37 from its value at the emission point. For low-loss dielectric materials such as soil, Dp is given by the following expression [22]: √ λ0 ε Dp = ·  , (2) 2π ε

3

where λ0 is the wavelength of the radiation in the free space (m) and ε  is dielectric constant (–). The internal electric field (E) variation generated by MW penetration into the medium is described by the following equation (E0 being the intensity of the incident electric field and z the MW propagation direction): Ez = E0 · e

− Dzp

.

(3)

Based on the above equations, in order to achieve the most effective and rapid heating, the highest possible frequency should be applied, but with the increasing electromagnetic wave frequency the radiation range decreases, and for values too high the radiation effect is noticeable only within a limited distance from the emission point. Therefore, the use of MWs at the frequency of 2.45 GHz results in a high and rapid heating but with a moderate penetration effect. In a schematic ex situ MW treatment plant, the polluted soil is loaded into the hopper and by means of a screw belt passes through the cavity of the plant where it is cleaned by MW irradiation [23] (Figure 1). The volatile organic compounds (VOCs) produced are treated in a dedicated offgas treatment line. MW heating was successfully applied to remove several contaminants from soil matrix. Experimental researches on polychlorinated biphenyls (PCBs)-contaminated soil

4

5

6

Figure 1. Schematic of ex situ continue microwave plant. (1) Hopper; (2) inlet screw belt; (3) contaminated soil; (4) magnetron (microwave source); (5) conveyor motor and (6) uncontaminated soil.

remediation was performed by several authors.[24–27] They found that an improvement in contaminant removal was obtained with the addition of energy absorbents, such as Cu2 O, MnO2 , NaOH, iron powder, graphite or granular activated carbons, and that rates of PCBs removed were highly dependent on MW power, soil moisture and the amount of adsorbent materials added. Yuan et al. [28] investigated the remediation of soil contaminated with hexachlorobenzene (HCB) using a domestic MW oven and powdered MnO2 as a MW absorber. Their results showed that a complete removal of HCB was obtained with 10 min of MW treatment. Similar results in terms of removal efficiency were also obtained by Kawala and Atamanczuk [19] in a pilot-scale study for the remediation of a tetrachloroethylene (TCE)-polluted soil where a MW power of 600 W was supplied intermittently for 75 h. After the treatment, the contaminant concentration decreased from 5000–22,300 mg kg−1 to 8–29 mg kg−1 . The MW treatment was also shown to be efficient in a short time for the remediation of soil polluted by polycyclic aromatic hydrocarbons (PAHs),[20,29] antibiotics [30] and crude oil.[31] Therefore, the literature findings highlight that MW remediation is very effective for a large number of polar and non-polar volatile and semi-volatile hydrocarbons and that the use of low-power generators for the supply of MW energy may help to reduce the costs of the full-scale remediation interventions. However, there is a lack of studies about the real possibility to apply a full-scale MW remediation treatment of hydrocarbon-polluted soils. Almost all studies reported data from laboratory-scale experiments, but very limited data about technical, energy and economical features of the MW treatment for full-scale activities have been shown, making unclear the limits and therefore the real applicability of the MW heating to remediate hydrocarbon-polluted soils.

Environmental Technology

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The objectives of this study are (i) to assess the influence of power treatment, heating duration and soil moisture on the temperature profiles generated by the MW irradiation, on the hydrocarbon residual contamination in soil and thus the percentage removal; (ii) to fit the contaminant residual concentration data with the kinetic model equation in order to calculate the desorption parameters useful for studying the desorption process and for predicting the response of the soil contamination treatment and (iii) to report technical, energy and economic assessment for the optimization of the treatment operating conditions of MW treatment systems. 2. Materials and methods 2.1. Soil contamination procedure A fine sandy soil (75–200 μm), free of anthropogenic contamination (Table 1) and commercially available diesel fuel (Esso, Italy) (Table 2) as pollutant were selected for the experiments. The soil was artificially contaminated by introducing a pollutant solution of diesel fuel in n-hexane (C6 H14 , purity 99%, Merck KGaA – Darmstadt, Germany) into a round-bottom flask containing the selected soil samples to obtain a representative contaminant concentration for a sandy soil.[13] Soil and pollutant solution were shaken for 48 h using an orbital shaker, then the n-hexane solvent was removed in 1 h, using a rotary evaporator, under slight vacuum, in order to obtain a homogeneous powdered soil. The contaminated soil was kept in a closed vessel and stored in a dark room at 4◦ C for 15 days then analysed by n-hexane Table 1. Physicochemical and granularity properties of the selected soil. Parameter

Value

Soil mineral pH Bulk density (g cm−3 ) Porosity (%) Specific surface area (m2 g−1 ) Hydraulic conductivity (cm s−1 ) Organic matter (g kg−1 ) Moisture content (%) Table 2.

Silica sand (75–200 μm) 8.72 1.36 32.5 3.45 5.0 × 10−2 3.55 0.0/8.0/12.0

3

extraction and subsequently gas chromatography (GC) for contaminant content before the MW treatment. Contamination procedure was carried out in triplicates and mean and standard deviation values of adsorbed contaminant concentration (C0 ) were calculated. After the contamination procedure, a number of soil samples were moisturized with deionized water to 8% and 12%. 2.2. Experimental apparatus and procedures An ex situ MW thermal treatment was simulated using a bench-scale apparatus carried out by modifying a 2.45 GHz domestic MW oven (Panasonic NN-GD458W Inverter – maximum power 1000 W) where the selected contaminated soil samples were irradiated (Figure 2). For the experiments, 20 g of each soil sample were placed into a quartz reactor (h 100 mm, internal ∅ 80 mm) positioned in the centre section of the oven cavity. The gas outlet section of the quartz reactor was connected to a condensing system, to a VOC capture system (activated carbon filters) and then to a vacuum pump to remove the contaminated gas in the reactor. Soil samples were treated by MW heating for a time of either 5, 10, 18, 30 or 60 min, using an applied power ranging from 100 to 1000 W. After a desired residence time, the soil temperature was measured with a sheltered type-k thermocouple axially inserted up to the middle of the soil sample. After the treatment, soil samples were removed from the apparatus, cooled at room temperature (20◦ C) and stored in a dark room at 4◦ C prior to analysis. The thermal treatment procedure was carried out in triplicates and mean values of contaminant residual concentrations (C) as a function of the treatment time were obtained for each selected power and soil moisture. For the calculation of C0 and C values, for each 20 g sample, a 2 g subsample was analysed for hydrocarbon concentration. The subsample was mixed with n-hexane in a Soxhlet extractor for 6 h. Five millilitres of effluent were mixed with 2 mL of n-hexane in a separate funnel, stirred

Properties of commercial diesel fuel used.

Parameter

Value

Density at 25◦ C (kg m−3 ) Flash point (◦ C) Water content (mg kg−1 ) Evaporation at 250◦ C (% v/v) Evaporation at 350◦ C (% v/v) Evaporation at 370◦ C (% v/v) n-Alkanes fraction C10 –C25 (%) n-Alkanes fraction C10 –C13 /C10 –C25 n-Alkanes fraction C14 –C17 /C10 –C25 n-Alkanes fraction C18 –C21 /C10 –C25 n-Alkanes fraction C22 –C25 /C10 –C25

900.6 55 200 64 85 95 39.9 37.1 41.7 16.7 4.5

(%) (%) (%) (%)

Figure 2. Schematic of bench-scale microwave apparatus. (1) Soil sample; (2) quartz reactor; (3) monitoring system with k-type thermocouple; (4) condenser; (5) condensate; (6) activated carbon filter and (7) vacuum pump.

P.P. Falciglia and F.G.A. Vagliasindi

for 2 min, and then left at rest for separation. The supernatant phase was mixed with internal standard (ISM-560 Ultra Scientific, USA) and analysed by GC. Due to their high proportion in diesel fuel, n-alkanes compounds (C10 –C25 ) were chosen as representative components,[3] and their total concentration in spiked and treated soil samples was taken as that of diesel fuel and expressed as mg kg−1 soil . The concentration of n-alkanes in soil samples was measured by GC (Agilent Technologies 6890 N) equipped with a mass spectrometer (Agilent Technologies 5975) using the US-EPA 8270-C method. A capillary column (HP-5, 30 m length × 0.32 mm ID × 0.25 μm film thickness) was used. The GC was operated with a helium-carrier-gas flow rate of 1.5 mL min−1 and the oven temperature programme starting at 40◦ C (held for 4 min) and increasing at a rate of 10◦ C min−1 to a maximum temperature of 310◦ C. The temperature of the injector was 270◦ C. After the contamination procedure, adsorbed diesel on soil (C0 ) as n-alkanes fractions (C10 –C25 ) for the spiked soil was 1916.4 mg kg−1 . Removal efficiency (R) was also calculated by the following expression:

C = C0 e−kt

(4)

3. Results and discussion 3.1. Contaminant removal

where C (mg kg ) is the residual concentration in soil after a treatment and C0 (mg kg−1 ) represents the initial contaminant concentration.

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Figure 3 shows the residual contaminant concentration adsorbed by the soil (C) and the soil temperature (T ) with

Kg–1)

C 8%

(5)

where C (mg kg−1 ) is the residual concentration in soil after a treatment time t (min), k (min−1 ) is rate of decay of the function and n is shape of the decay curve. k is temperature dependent. Residual concentration (C) results obtained during the experiments were fitted using the first-order kinetic model expressed by Equation (5). Desorption parameters k and n were calculated for each soil moisture value at different tested temperatures and the correlation was assessed as correlation coefficient r 2 . Obtaining the experimental parameters k and n, Equation(5) could represent a valuable tool in calculating residual concentration C or desorption rate at any given initial contaminant concentration in soil and in identifying the specific power and the remediation time required to reach specific targeted levels of remediation.

−1

C 0% 2000

n

1400 1200 1000 800 600 400 200 0

T 12% 260 240 220 200 180 160 140 120 100 80 60 40 20 0 70 T (°C)

C0 − C · 100, C0

2.3. Kinetic data modelling Residual hydrocarbon concentration curves as a function of the desorber residence time follow a first-order kinetic,[13] defining an exponential decay:

T 12% 260 240 220 200 180 160 140 120 100 80 60 40 20 0 70

T (°C)

R% =

C (mg Kg–1)

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4

0

10

20

30 40 Time (min)

50

60

Figure 3. Residual contaminant concentrations (C) and temperature (T ) profiles of soil (soil moisture 0%, 8% and 12%) with time at powers of 250, 440, 600 and 1000 W.

Environmental Technology Table 3.

5

Removal efficiency (R) of the MW treatment for the different powers and times investigated.

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R (%) Time (5 min)

Time (10 min)

Time (18 min)

Time (30 min)

Time (60 min)

P (W)

0%

8%

12%

0%

8%

12%

0%

8%

12%

0%

8%

12%

0%

8%

12%

100 250 440 600 1000

11.2 15.1 19.0 24.8 27.5

16.5 25.4 38.5 47.8 55.1

22.5 34.5 44.6 52.8 58.0

15.1 21.3 34.8 42.1 47.8

21.1 33.8 53.0 59.8 65.0

25.2 41.4 57.5 65.1 68.0

18.6 27.5 43.1 58.2 62.9

24.5 39.9 60.8 69.7 73.9

28.7 47.2 67.5 73.7 77.2

22.8 32.9 55.1 69.1 75.3

27.8 47.8 68.6 79.1 80.6

31.4 54.0 73.3 81.2 86.9

27.1 47.9 75.4 86.9 89.5

31.1 59.3 86.9 91.6 94.2

36.7 63.4 88.5 91.6 94.8

time, at different powers. As expected, contaminant concentration in the soil after MW heating decreased with time for all the soils, and the rate of diesel desorption increased with the power applied increasing. For all the water contents investigated, minimal C values (less than 200 mg kg−1 ) were achievable only when applying the power of 600 W for a treatment duration longer than 60 min. The lowest C reached was observed for the wet soils (12% and 8% water content) and this highlights that soil moisture is the main key factor of the remedial process. On the other hand, residual diesel concentration strictly depends on the maximum temperature that the soil can reach during the MW treatment. As expected, T increased with increasing time and P for all the samples of soil with different moisture contents investigated. In all cases, T increased rapidly at the beginning of the treatment and it stabilized after about 20 min, and it can be clearly seen that at higher P, T rose more rapidly. For the lowest P values investigated, a slight increase in T was observed. For P higher than 440 W, T rose up to 184◦ C, whereas the maximum increase of 240◦ C (T = 260◦ C) was reached for the 1000 W treatment. The increase in T in time is linearly proportional to the dielectric constant defined loss factor (ε  ) that denotes the ability of the soil to convert electromagnetic energy into heat, due to the dielectric polarization of the particles in an alternating electric field. The temperature profile rises more rapidly at the beginning and tends to stabilize after a fixed time due to the reducing ability of the MW absorbent medium (soil) to convert energy into heat with increasing heat absorbed, and therefore with increasing temperature. This specific behaviour is due to the fact that ε decreases with T increasing and its value is close to zero for T values higher than 200◦ C.[32] The highest T values found for the soil with water content depends on the improvement of dielectric properties of the medium containing water, which is an excellent MW absorber.[33] Therefore, the presence of water in soil results in a significant increase in soil temperature and therefore in the best contaminant removal process. Temperature profiles obtained are in agreement with other literature findings.[30,31,34] Residual concentration

values observed are in agreement with other results obtained by previous authors on the conventional thermal desorption treatment. In our previous study,[13] we found that a soil temperature of 175◦ C was sufficient to remedy a dieselpolluted fine sandy soil at the final diesel concentration of about 150 mg kg−1 and that temperatures higher than 250◦ C are necessary in order to obtain residual concentration less than 10 mg kg−1 . Lee et al. [35] also showed that at about 300◦ C n-hexadecane and diesel can be removed completely from a fine sandy soil. Based on residual diesel concentration adsorbed in soil, contaminant removal efficiency (R) vs. time was also calculated for all soils and values are given in Table 3. Maximum R values were observed for soils with 8% and 12% water content. For wet soils, good efficiency higher than about 70% was reached also at power higher than 600 W for a duration longer than 30 min. The maximum R value of about 95% was reached for both the wet soils treated at 1000 W for 60 min. (i.e. 76% at 100◦ C and 95% at 150◦ C). A lower value of 90% was reached for dry soil at the same operating conditions. For duration shorter than 18 min, R was lower than 77% (i.e. 77% for 12% of soil moisture, 74% for 8% and 63% for soils with 0% water content). It is important to highlight that, at the highest powers, especially for the first 10 min, a significant contaminant removal increase was observed for the wet soils compared with dry soil, due to the evaporation and contaminant stripping phenomena. In fact, despite the soil temperature being in the same range for all soils, a significant difference in terms of contaminant removal was recorded. This was also observed for the treatments at the lowest powers of 100 and 250 W, for which, despite the soil temperature being about 100◦ C, removals up to 60% were achieved. For soils contaminated by polar compounds, this phenomena could be the effect of a selective heating but for non-polar contaminants as diesel this could be ascribable just to a distillation process.[19,20] Overall, R values obtained in the performed experiment are in the same range as that reported in the literature on the MW treatment of hydrocarbon-polluted soil, but the extremely different operating conditions adopted and the several dielectric materials used make a direct and effective comparison of the results very difficult.[31]

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P.P. Falciglia and F.G.A. Vagliasindi 100 W

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1000 800 600 200

60

y = 0.0003x + 0.0883 R² = 0.8347 y = 0.0003x + 0.0519 R² = 0.9377

0.3 0.25 0.2

y = 0.0001x + 0.0275 R² = 0.8145

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Figure 4. Residual contaminant concentrations (C) with time and first-order kinetic model curves at powers 100, 250, 440, 600 and 1000 W (a)–(c) and the rate of decay (k) (d) as a function of treatment power for 0, 8.0 and 12.0% soil moisture.

Huang et al. [27] demonstrated that the maximum removal efficiency, for a soil polluted by PCBs at 5 mg kg−1 and treated at 800 W for a period of 45 min, is about 95%. Very high PCBs removal was also found by Abramovitch et al. [24] investigating several operating conditions and the properties of the materials. Removal efficiency of crude oil contaminant of 95% (initial contaminant concentration in soil = 7.8%) was observed for a 15 min of 800 W treatment enhanced by different MW absorbers, such as activated carbon powder or graphite fibres.[32] Contaminant removal higher than 90% was also found by Lin et al. [30], Calvert and Suib,[36] Robinson et al. [20] and Kawala and Atamanczuk,[19] in studies aimed at investigating the effects of MW heating on soils polluted with chloramphenicol, HCB, PAHs and TCE, respectively, in treatments enhanced by several MW absorbing materials or solutions. However, high removal efficiencies comparable to the MW treatment can be reached by using soil washing with surfactants,[3] or remediation treatments, such as electrokinetic decontamination,[10] electrokinetic-Fenton oxidation [9] or conventional thermal desorption remediation, [13] which require a higher energy consumption and consequently a higher cost. Li et al. [37] reported that for a sandy soil spiked with diesel fuel at different rates (ranging from 500 to 50000 mg kg−1 ) the total petrol hydrocarbons (TPHs) natural biodegradation, for an incubation period of 110 d, reached the maximum value in the range of 70–73%. £ebkowska et al. [38] found an 80% removal of TPH for a sandy soil polluted by diesel at 5300 mg kg−1 of

TPH treated by ex situ biopile remediation for a period of about 2 months. Sprocati et al. [39] carried out a study on bioaugmentation aimed at the remediation of a soil co-contaminated (spiked) with both diesel oil and heavy metals, using microcosms in different experimental conditions. They reported an effective removal of diesel oil close to 75%. Do et al. [7] showed that an in situ chemical oxidation treatment of a diesel-polluted soil at 5000 mg kg−1 , using peroxymonosulphate/cobalt, was characterized by the maximum contaminant degradation of approximately 47% and that a sequential injection treatment using a large quantity of chemicals was needed to reach a contaminant degradation of 88%. Low diesel removal efficiency in the range 27–70% was obtained by Lee et al. [35], Fernández et al. [5], Couto et al. [40] and Wei et al. [41] 3.2. Kinetic data modelling Residual concentration data were fitted to Equation (5), and values for the correlation coefficient (r 2 ) were calculated. Fitting curves are shown in Figure 4(a)–(c), whereas model parameters (rate of contaminant decay k and shape of the curve n and r 2 values) are reported in Table 4. The firstorder kinetic model showed an excellent correlation with the experimental data according to the very high r 2 values. k increased with increasing power for all the soils due to the nature of the thermal process (Figure 4(d)). The slopes of the k trend were different for the three soils treated but similar values were observed for both the wet soils. The different slopes observed for the soils with diverse water contents imply that the activation energy is correlated with the

Environmental Technology

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Table 4. Desorption parameters (k, n) and correlation coefficient (r 2 ) of the first-order kinetic model at powers of 100, 250, 440, 600 and 1000 W for all tested soils. 0%

0.381 0.623 0.715 0.762 0.710

0.992 0.976 0.991 0.997 0.996

k

0.123 0.145 0.207 0.277 0.369

characteristics of the soil. Specifically, the lowest activation energy is required for moist soils, while the highest for dry soils. This is due to the highest dielectric properties of the soils containing water that results in an improvement of the performance of the heating treatment.[42] Knowing the residual concentration values as a function of treatment temperature and treatment duration could be significant in assessing the change in energy efficiency and cost of a full-scale thermal remedial process.

n

r2

0.278 0.430 0.533 0.517 0.458

0.993 0.990 0.988 0.994 0.988

12% k

(min−1 ) 0.142 0.226 0.270 0.327 0.380

r2

0.290 0.355 0.491 0.509 0.654

0.992 0.989 0.993 0.999 0.998

Dp

e ''

e'

(a)

n

120

2.40 2.20

100

2.00 1.80 1.60

80

1.40 1.20

60

1.00 0.80

40

D p (cm)

0.068 0.053 0.074 0.092 0.128

n

(min−1 )

0.60 0.40

20

0.20 0.00

3.3. Technical, energy and economic considerations Evaluating the scheme reported in Figure 1, it is evident that the thickness of the soil layer and the speed of the screw belt represent key factors of the remedial process, due to the limitations of the MW penetration through the soil. This, for the capability of the electromagnetic wave at the frequency of 2.45 GHz to penetrate the soil depth, is clearly expressed by Equations (2) and (3). The dielectric constants (ε  , ε ) and deep penetration (Dp ) values were assessed for different specific operating powers (P), namely 12.5, 22.0, 30 and 50 kW kg−1 (Figure 5(a)), considering the maximum temperature of the soil (moisture 12%) generated by MW irradiation and the dielectric constant-temperature relationship of a hydrocarbon-polluted soil.[27] The changing of the internal electric field in the soil (E) generated by the MW application as a function of the soil thickness was also calculated by Equation (3) and showed in Figure 5(b). Data obtained show that the application of a specific power of about 10 kW kg−1 results in Dp of about 50 cm. Higher Dp values, up to about 1 m, are reachable for higher powers. Therefore, for the application of a conventional specific power of about 30– 50 kW kg−1 , a soil layer not higher than 30 cm should be considered. Based on desorption parameters (k, n) reported in Table 4 and considering a C0 value ranging from 750 to 5000 mg kg−1 , the residual concentration C values after different remediation times and the minimal specific energy required E (kWh kg−1 ) to reach the final remedy target of C = 500 mg kg−1 were calculated. The range 750 to 5000 mg kg−1 was selected for C0 since it represents typical wide range of soil diesel pollution values found in real commercial or industrial sites. The value 500 mg kg−1 was

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2

2.2

Thickness (m)

Figure 5. (a) Dielectric constants (ε , ε ) and deep penetration (Dp ) values as a function of applied specific power P. (b) Changing of internal electric field into the soil (E) generated by microwave application as a function of the soil thickness.

selected since it could represent a typical remediation target in terms of contaminant residual concentration. Finally, considering an energy cost of 0.00046 ¤k−1 W−1 h−1 , the specific energy costs CE (¤ ton−1 ) for the soil treatment was assessed. The variation of E and CE values vs. C0 is reported in Figure 6. Results show a linear increase in E and CE with C0 increasing. The E range observed is 5–45 kWh kg−1 for C0 from 1500 to 5000 mg kg−1 . Similar E values were observed for 8% and 12% soil moisture, whereas, significantly, the highest values were found for the dry soil (moisture 0%). This behaviour is reflected in the cost variation with C0 . CE calculated are in the range of 30–220 ¤ ton−1 for C0

8

P.P. Falciglia and F.G.A. Vagliasindi E (0%) C (0%)

E (8%) C (8%)

E (12%) C (12%) 240 220

r² = 0.938

180

200

160 r² = 0.9567

140 120

r² = 0.9624

100

160 140 120 100

80

80

60

r² = 0.938

40 20 0 500

180

r² = 0.9567

r² = 0.9624 1500

2500

3500

4500

5500

60

Energy costs, CE (€ton–1)

Specific energy required, E (KWh Kg–1)

200

40 20 0

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C0(mg Kg–1)

Figure 6. Specific energy required (E) and energy cost (CE ) as a function of initial contaminant concentration (C0 ) for the MW treatment of a diesel-polluted soil at different contaminant concentration levels.

higher than 1500 mg kg−1 . The presence of humidity in the soil determines an average CE reduction from about 25 to 70 ¤ ton−1 , representing a critical key factor for the optimization of a full-scale MW treatment. Specific energy data calculated are in agreement with Mavrogianopoulos et al.[43] Moreover, in our previous calculation [13] and in other works [44] an average cost of fuel consumption ranging from 200 to 400 ¤ ton−1 is reported for hydrocarbon-polluted soil remediation by conventional ex situ thermal desorption with the rotary kiln system. Hence, the MW treatment could represent a suitable cost-effective alternative to conventional ex situ thermal desorption for the remediation of hydrocarbon-polluted soil. 4.

Conclusions

This paper reports a research aimed at investigating the real applicability of a MW heating treatment to remediate hydrocarbon-polluted soil. Based on data obtained by the laboratory-scale activity, a technical, energy and economic assessment for the optimization of the treatment was carried out. The following conclusions have been drawn based on results and discussion: • Soil moisture significantly influences the contaminant removal processes during the treatment. Residual contaminant concentrations lower than 200 mg kg−1 were achievable only when applying powers higher than 600 W for a treatment duration longer than 60 min. Specifically, for remediation times shorter than 10 min the effect of the distillation process increases the contaminant removals (about 60%) for wet soils, whereas for times longer than 10 min soil temperature is the main key factor in the remedial process. • The first-order kinetic model showed an excellent correlation (r 2 > 0.976) with the experimental data

for residual concentration at all operating powers and for all soil moistures tested. Experimental parameters k and n calculated could be useful, representing a valuable tool in calculating residual concentration or desorption rate at any given initial condition. • The specific energy costs calculated for the MW treatment are in the range of 30–220 ¤ ton−1 for soil pollutions higher than 1500 mg kg−1 , whereas an average cost of fuel consumption ranging from 200 to 400 ¤ ton−1 is needed for hydrocarbon-polluted soil remediation by conventional ex situ thermal desorption with rotary kiln systems. The use of MW heating with respect to a conventional ex situ thermal desorption treatment could significantly decrease the energy consumption needed for the removal of hydrocarbon contaminant from polluted soils, and a further decrease of about 25% is achievable in the cases of moisturized soils. Therefore, the MW treatment could represent a suitable cost-effective alternative to the conventional thermal treatment for the remediation of hydrocarbon-polluted soil. References [1] Fan MY, Xie RJ, Qin G. Bioremediation of petroleumcontaminated soil by a combined system of biostimulation– bioaugmentation with yeast. Environ Technol. 2014;35(4): 391–399. [2] Pazos M, Alcantara MT, Rosales E, Sanroman MA. Hybrid technologies for remediation of diesel fuel polluted soil. Chem Eng Technol. 2011;34(12):2077–2082. [3] Khalladi R, Benhabiles O, Bentahar F, Moulai-Mostefa N. Surfactant remediation of diesel polluted soil. J Hazard Mater. 2008;164:1179–1184. [4] Serrano A, Gallego M, González JL, Tejada M. Natural attenuation of diesel aliphatic hydrocarbons in contaminated agricultural soil. Environ Pollut. 2008;151:494–502. [5] Fernández MD, Pro J, Alonso C, Aragonese P, Tarazona JV. Terrestrial microcosms in a feasibility study on the remediation of diesel-contaminated soils. Ecotoxicol Environ Saf. 2011;74:2133–2140. [6] Chien Y. Field study of in situ remediation of petroleum hydrocarbon contaminated soil on site using microwave energy. J Hazard Mater. 2012;199–200:457–461. [7] Do SH, Jo JH, Jo YH, Lee HK, Kong SH. Application of peroxymonosulfate/cobalt (PMS(Co(II)) system to treat diesel-contaminated soil. Chemosphere. 2009;77: 1127–1131. [8] Bento FM, Camargo AO, Okeke BC, Frankenberger WT. Comparative bioremediation of soil contaminated with diesel oil by natural attenuation biostimulation and bioagumentation. Bioresour Technol. 2005;96:1049–1055. [9] Tsai TT, Sah J, Kao CM. Application of iron electrode corrosion enhanced electrokinetic-Fenton oxidation to remediate diesel contaminated soils: a laboratory feasibility study. J Hydrol. 2010;380:4–13. [10] Pazos M, Plaza A, Martín M, Lobo MC. The impact of electrokinetic treatment on a loamy-sand soil properties. Chem Eng J. 2012;183:231–237. [11] Chang YYg, Roh H, Yang JK. Improving the clean-up efficiency of field soil contaminated with diesel oil by the application of stabilizers. Environ Technol. 2013;34(11): 1481–1487.

Downloaded by [Pietro Paolo Falciglia] at 11:31 02 April 2014

Environmental Technology [12] Liu PWG, Wang SY, Huang SG, Wang MZ. Effects of soil organic matter and ageing on remediation of diesel-contaminated soil. Environ Technol. 2012;33(23): 2661–2672. [13] Falciglia PP, Giustra MG, Vagliasindi FGA. Lowtemperature thermal desorption of diesel polluted soil: influence of temperature and soil texture on contaminant removal kinetics. J Hazard Mater. 2011;185:392–400. [14] Xu DP, He YL, Zhunag XN, Gu QB. Desorption kinetics of DDTs from contaminated soil during processes of thermal desorption. Res Environ Sci. 2013;26(2):202–207. [15] Gao YF, Yang H, Zhan XH, Zhou LX. Scavenging of BHCs and DDTs from soil by thermal desorption and solvent washing. Environ Sci Pollut Res. 2013;20(3):1482–1492. [16] Tatáno F, Felici F, Mangani F. Lab-scale treatability tests for the thermal desorption of hydrocarbon-contaminated soils. Soil Sediment Contam. 2013;22:433–456. [17] Barba AA, Acierno D, d’Amore M. Use of microwaves for in-situ removal of pollutant compounds from solid matrices. J Hazard Mater. 2011;207–208:128–135. [18] Robinson JP, Snape CE, Kingman SW, Shang H. Thermal desorption and pyrolysis of oil contaminated drill cuttings by microwave heating. J Anal Appl Pyrol. 2008;81:27–32. [19] Kawala Z, Atamaczuk T. Microwave-enhanced thermal decontamination of soil. Environ Sci Technol. 1998;32:2602–2607. [20] Robinson JP, Kingman SW, Snape CE, Shang H, Barranco R, Saeid A. Separation of polyaromatic hydrocarbons from contaminated soils using microwaves heating. Sep Purif Technol. 2009;69:249–254. [21] Clark DE, Folz DC, West JK. Processing materials with microwave energy. Mater Sci Eng. 2000;A287:153–158. [22] Acierno D, Barba AA, d’Amore M, Pinto IM, Fiumara V. Microwaves in soil remediation from VOCs. 2: buildup of a dedicated device. Environ Energy Eng. 2004;50:722–732. [23] Ha S, Choi K. A study of a combined microwave and thermal desorption process for contaminated soil. Environ Eng Res. 2010;15:225–230. [24] Abramovitch RA, Bangzhou H, Davis M, Peters L. Decomposition of PCB’s and other polychlorinated aromatics in soil using microwave energy. Chemosphere. 1998;37: 1427–1436. [25] Liu X, Yu G. Combined effect of microwave and activated carbon on the remediation of polychlorinated biphenylcontaminated soil. Chemosphere. 2006;63:228–235. [26] Liu X, Zhang Q, Zhang G, Wang R. Application of microwave irradiation in the removal of polychlorinated biphenyls from soil contaminated by capacitor oil. Chemosphere. 2008;72:1655–1658. [27] Huang G, Zhao L, Dong Y, Zhang Q. Remediation of soils contaminated with polychlorinated biphenyls by microwave-irradiated manganese dioxide. J Hazard Mater. 2011;186:128–132. [28] Yuan S, Tian M, Lu X. Microwave remediation of soil contaminated with hexachlorobenzene. J Hazard Mater. 2006;B137:878–885. [29] Abramovitch RA, Bangzhou H, Abramovitch DA, Jiangao S. In situ decomposition of PAHs in soil and desorption of

[30] [31] [32]

[33]

[34] [35] [36]

[37]

[38]

[39]

[40]

[41]

[42] [43] [44]

9

organic solvents using microwaves energy. Chemosphere. 1998;39:81–87. Lin L, Yuan S, Chen J, Wang L, Wan J, Lu X. Treatment of chloramphenicol-contaminated soil by microwave radiation. Chemosphere. 2010;78:66–71. Li D, Zhang Y, Quan X, Zhao Y. Microwave thermal remediation of crude oil contaminated soil enhanced by carbon fiber. J Environ Sci. 2009;21:1290–1295. Robinson JP, Kingman SW, Lester EH, Yi C. Microwave remediation of hydrocarbon contaminated soils – scale up using batch reactors. Sep Purif Technol. 2012;96: 12–19. Hallikainen MT, Ulaby FT, Dobson MC, El-Rayes MA, Wu LK. Microwave dielectric behavior of wet soil – part 1: empirical models and experimental observations. IEEE Transact Geosci Remote Sens. 1985;GE-23:25–34. Diprose MF. Some considerations when using a microwave oven as a laboratory research tool. Plant Soil. 2001;229: 271–280. Lee JK, Parka D, Kimb B, Dongb J, Lee S. Remediation of petroleum-contaminated soils by fluidized thermal. Waste Manage. 1998;18:503–507. Calvert CA, Suib SL. An initial study into the use of microwave remediation of hexachlorobenzene treated soil using selected oxidants and coated graphite rods. J Soil Sediment. 2007;7:147–152. Li H, Zhang Y, Kravchenko I, Xu H, Zhang CG. Dynamic changes in microbial activity and community structure during biodegradation of petroleum compounds: a laboratory experiment. J Environ Sci. 2007;19:1003–1013. Łebkowska M, Zborowska E, Karwowska E, MiaskiewiczPeska E, Muszynski A, Tabernacka A, Naumczyk J, Jeczalik M. Bioremediation of soil polluted with fuels by sequential multiple injection of native microorganisms: field-scale processes in Poland. Ecol Eng. 2011;37:1895–1900. Sprocati AR, Alisi C, Tasso F, Marconi P, Sciullo A, Pinto V, Chiavarini S, Ubaldi C, Cremisini C. Effectiveness of a microbial formula as a bioaugmentation agent tailored for bioremediation of diesel oil and heavy metal cocontaminated soil. Process Biochem. 2012;47:1649–1655. Couto MNPFS, Pinto D, Basto MCP, Vasconcelos TSD. Role of natural attenuation phytoremediation and hybrid technologies in the remediation of a refinery soil with old/recent petroleum hydrocarbons contamination. Environ Technol. 2012;33(18):2097–2104. Wei J, Liu X, Zhang X, Chen X, Liu S, Chen L. Rhizosphere effect of Scirpus triqueter on soil microbial structure during phytoremediation of diesel-contaminated wetland. Environ Technol. 2014;35(4):514–520. Acierno D, Barba AA, d’Amore M. Microwaves in soil remediation from VOCs. 1: heat and mass transfer aspects. Environ Energy Eng. 2003;49:1909–1921. Mavrogianopoulos GN, Frangoudakis A, Pandelakis J. Energy efficient soil disinfestation by microwaves. J Agric Eng Res. 2000;75:149–153. US-EPA. Remediation technology cost compendium – year 2000. Solid Waste and Emergency Response; 2004. (EPA542-R-01-009).